This invention relates to a speed-changing device for a bicycle, and more particularly to a speed-changing device comprising a rear derailleur and a control wire therefor which is operated to actuate the derailleur to switch a driving chain to a selective one of a plurality of sprockets.
Generally, the rear derailleur comprises a pantograph mechanism comprising a supporting member supported to a bracket, two linkage members, and movable member, these four members being pivotally connected through four parallel spindles. A chain change-over cage having two pulleys is pivoted to the movable member through one horizontal spindle.
A control cable (Bowden cable) for operating the derailleur comprises an inner wire and an outer sheath guiding the wire, the inner wire being fixed at its one end to one member, for example, one linkage member, among the four members at the pantograph mechanism and the outer sheath being fixed at its one end to another member, for example, the supporting member, movable relative to the wire fixing member. The wire is operated to transform the pantograph mchanism and shift the movable member axially of the multi-stage sprockets at a hub of the bicycle to thereby switch a driving chain to a selective one of the sprockets through the pulleys.
In the above construction, the control wire, which is guided by the outer sheath follows a desired route connecting, the derailleur and a control lever therefor without being under tension. The outer sheath, when used, considerably projects laterally of a wheel of the bicycle and may catch a cyclists' foot or the like. Also, the outer sheath causes poor transfer efficiency of the lever control for the derailleur, and expands or contracts in length more than the control wire causing a change in the relative effective length therefore and an improper speed-changing operation.
Two members in relative movement at the pantograph mechanism hold the control wire and outer sheath at respective one ends thereof, whereby the wire is operated to transform the pantograph mechanism and move the movable member with respect to the supporting member only axially of the multi-stage sprockets. Consequently, the wire, when pulled for changing the speed, allows the pulleys carried by the movable member to shift only axially of the sprocket, making them difficult to control. Furthermore, when the chain is stationary, i.e., no pedalling, the derailleur is subjected to even greater resistance to its movement, whereby the speed-changing is impossible.
The latter problem is solvable by use of a saver mechanism provided at the pantograph mechanism, the saver mechanism comprising an energy conserving plate separate from the four pantograph members and an energy conserving spring acting on the plate, the plate holding the wire or outer sheath, so that when resistance over a predetermined value is applied against the speed-changing, the wire is controlled to shift only the energy conserving plate against the spring, whereby the chain, when the resistance is eliminated, is automatically switched to the desired speedchange stage. However, the saver mechanism results in a high cost of manufacture and an increase in weight.
This invention has been designed to overcome the above problem of the conventional derailleur. An object of the invention is to provide a speed-changing device capable of controlling a derailleur by use of a control wire only without an outer sheath to thereby eliminate the defects caused by the outer sheath and improve the speed change efficiency. The wire also is controllable even when resistance greater than the predetermined value is applied against the speed-changing, and the chain, when the resistance decreases, can be switched to a desired speed-change stage defined by operating the wire. Furthermore, there is no fear that a cyclist may catch his foot or the like by the conventional outer sheath projecting sidewise of the bicycle wheel.
The speed-changing device of the invention is provided with a wire guide uniquely positioned with respect to the derailleur so that a control wire without an outer sheath is guided through the wire guide to thereby control the pantograph mechanism of the derailleur. The wire is operated to shift the pulleys at the change-over cage carried by the movable member, axially and simultaneously radially of the sprockets.
In greater detail, the pantograph mecanism comprising a supporting member, two linkage members and movable member, is pivoted through a first horizontal spindle swingably to a bracket fixed to a fork end of the bicycle. A wire holder is provided at one of the two linkage members and movable member to thereby hold one end of the control wire. A wire guide for guiding the wire to the wire holder is disposed at a position outward with respect to the common axis of the sprockets and beyond a phantom straight line connecting the axes of the wire holder and first horizontal spindle.
When the wire is operated to transform the pantograph mechanism to thereby shift the pulleys axially of the sprockets, the pantograph mechanism simultaneously swings with respect to the bracket around the first horizontal spindle to thereby shift the pulleys also radially of the sprockets.
Accordingly, the speed-changing device of the invention is capable of only changing the speed by use of the control wire without requiring an outer sheath thereby improving the transfer efficiency of the lever control for the derailleur. Furthermore, due to the absence of an outer sheath, there is no fear that expansion of contraction of its length will change the relative length to the control wire to thereby cause a poor speed-changing operation.
Furthermore, the speed-changing device of the invention, when the wire is pulled for changing the speed, not only transforms the pantograph mechanism but also swings it with respect to the bracket to thereby shift two pulleys radially as well as axially of the sprockets. Hence, the driving chain guided by the pulleys is smoothly switched to a desired sprocket from the oblique direction thereof resulting in shifting with a light control of the wire. In addition, even when the chain is stationary during the absence of pedalling and resistance more than the predetermined value is applied against axial movement of the pulleys, the control wire is operable to swing the pantograph mechanism through the supporting member, so that energy from the wire control can be conserved and the chain, when the resistance is released, can be switched to a sprocket at a desired speed-change stage. As a result, one saver mechanism, such as the conventional energy conserving spring or plate, is not necessary, whereby the speed-changing device of the invention is inexpensive while still attaining the above described advantages. Furthermore, absence of the outer sheath eliminates the possibility of a cyclist catching his foot or the by the projecting outer sheath.
Alternatively, the wire guide may be mounted to the supporting member or bracket of the derailleur, the fork end of the bicycle to which the bracket is fixed, or a hub shaft fixed to the fork end supporting the sprockets.
These and other objects and novel features of the invention will be more apparent from the description of an embodiment of the invention in accordance with the drawings.